1. Field of the Invention
The present invention relates to an automatic focus-adjusting apparatus for detecting the focusing position of a lens by analyzing a frequency component provided from an image in a video camera, a still camera, etc. and automatically adjusting a focus of the lens.
2. Description of the Related Art
In a known automatic focus-adjusting apparatus, a specified frequency component is selected from a video signal outputted from an image pickup element in a video camera, a still camera, etc. The focus of a lens is adjusted by moving the lens forward or backward such that this specified frequency component is maximum.
In this automatic focus-adjusting apparatus, a lens system is considered to be a kind of a low-pass filter. An equivalent frequency band width with respect to a photographed object separated from the camera by a constant distance is changed by adjusting the focus of the lens. A change in amplitude of a high frequency component of the video signal close ot a cutoff frequency is detedted by the above change in band width to detect definition of a picture, thereby providing the focusing position of the lens. In this case, the amplitude of the high frequency component close to the cutoff frequency becomes maximum in a lens position in which an image formed by the lens is best. The amplitude of the high frequency component is reduced as a focal position of the lens is shifted from this best image forming position forward or backward.
Concretely, a signal having a specified high frequency component (for example, 1 MHz) is taken by using a band-pass filter out of the video signal provided by scanning an image in a horizontal direction. The position of the lens is adjusted to focus the lens such that an amplitude of this high frequency component is maximum. Such an automatic focus-adjusting apparatus is shown in e.g., Japanese Patent Application Laying Open (KOKAI) No. 62-146081.
In this system for detecting large or small values of the amplitude of the specified high frequency component provided through the band-pass filter, the video signal is obtained by sequentially scanning the image in the horizontal direction. Accordingly, the accuracy in focusing operation with respect to a vertical image is high, but the accuracy in focusing operation with respect to a horizontal image is low. Therefore, for example, it is impossible to preferably focus the lens with respect to an image having a transversally striped pattern such as blinds of a window.
The image has two-dimensional information, but is converted to a one-dimensional time series video signal by sequentially scanning the image in the horizontal direction. In the case of an image of the photographed object having a longitudinally striped pattern with edges in the horizontal direction, an obtained brightness signal includes an extremely high frequency component in a position indicative of this signal on a scanning line. It is possible to take a signal having the specified high frequency component out of this brightness signal through the band-pass filter. However, in the case of an image of the photographed object having a transversally striped pattern with edges in the vertical direction, an obtained brightness signal does not include many high frequency components. Accordingly, no high accuracy in focusing operation is obtained in a general system for providing the focusing position of the lens by monitoring an amplitude of the specified high frequency component.